Today, a steam turbine using steam at a temperature of about 600° C. has been put into practical use in view of improving the turbine efficiency. In addition, it is now being studied and developed to raise the steam temperature to 650° C. or more in order to improve the turbine efficiency.
Since the above steam turbine uses the steam of high temperature, it is necessary to use a heat-resistant alloy resistive to that temperature. But, such a heat-resistant alloy is used in a limited range because it is expensive, and it is difficult to produce large-size parts.
For example, a steam inlet pipe such as an inlet sleeve where high temperature steam flows is made of a Ni-base alloy or the like having excellent heat resistance, and seal rings contacted to the steam inlet pipe and casings contacted to the seal rings are formed of CrMoV steel or 12Cr steel.
As described above, the main steam of high temperature flows through the inlet sleeve, and heat is conducted to the casings via the seal rings which are contacted to the inlet sleeve. Therefore, the temperatures of the seal rings and the casings are raised as the main steam increases to a high temperature, and it is necessary to take measures against heat when a conventional material is used to configure the seal rings and the casings.